The present invention relates to hollow blow-molded containers of a biaxially oriented thermoplastic material, and more particularly to thin-walled plastic containers configured to accommodate partial evacuation without adverse effects on their apperance.
Lightweight, thin-walled containers made of thermoplastic materials such as polyester resin and thermoplastic polymers containing at least 50% by weight polymerized nitrile-groups-containing monomer (hereinafter "nitriles") are well known in the container industry. For example, polyethylene terephthalate (PET) has a wide range of applications in the field of containers for foodstuffs, flavoring materials, cosmetics beverages and so on. PET can be molded, by orientation-blowing, into transparent thin-walled containers having a high stiffness, impact strength and improved hygienic qualities with a high molding accuracy. Strong, transparent and substantially heat resistant containers may be produced by the biaxial-orientation blow-molding process in which a parison is oriented both laterally and longitudinally in a temperature range suitable for such orientation. Nitrile and heat-set PET containers are particularly heat resistant. Biaxially-oriented blow-molded containers have greater stiffness and strength as well as improved gas barrier properties and transparency.
When a thermoplastic container is filled with a hot liquid (such as a liqud sterilized at a high temperature) and sealed, subsequent thermal contraction of the liquid upon cooling results in partial evacuation of the container which tends to deform the container walls. Backflow into a filling mechanism and the use of vacuum filling equipment during filling operations can similarly creat a partial vacuum inside the container resulting in its deformation. Such deformation typically concentrates at the mechanically weaker portions of the container, resulting in an irregular and commercially unacceptable appearance. Further, if the deformation occurs in an area where the label is attached to the container, the appearance of the label may be adversely affected as a result of container deformation.
By increasing the wall thickness of the container it is possible to some extent to strengthen the container walls and thus decrease the effects of vacuum deformation. However, increasing the wall thickness results in a substantial increase in the amount of raw materials required to produce the container and a substantial decrease in production speed. The resultant increased costs are not acceptable to the container industry. Additionally, increase in wall thickness results in decrease in bottle fill capacity.
A prior attempt to reduce the effects of vacuum deformation is disclosed in U.S. Pat. No. 3,708,082 to Platte. Platte discloses a container with four flat wall-panels comprising the body portion of the container. A rib circumscribes the entire container in a region below the handle and serves to rigidify the side wall-portions in a circumferential direction. The rib also acts as a hinge to allow limited inward collapsing of the container along selected regions.
Another prior approach to reduction of the effects of vacuum deformation is disclosed in Japanese Patent Application No. 54-30654. In this approach, a container is provided with a plurality of recessed collapse panels, separated by lands, which allow uniform controlled inward deformation so that vacuum effects are accommodated in a uniform manner without adverse effects on the appearance of the container.
U.S. Pat. No. 4,298,045 to Weiler et al. shows another prior art approach in which a container has rigidifying grooves and embossments provided in the side walls of the container. Rather than controlling collapse, these rigidifying features substantially eliminate collapse, and are thus useful only with relatively low levels of evacuation.
Prior art approaches have included the use of collapse panels (i.e., indented surface areas which provide for controlled, quantified collapse) to overcome thermal deformation; however, problems have developed in containers designed with collapse panels. While collapse panels accommodate a great degree of controlled deformation, as the vacuum inside the containers increases, more and more collapse is required from the collapse panels without permitting collapse of the intervening lands. By increasing the length of the corner step of the collapse panels the rigidity of the lands may be increased. See FIGS. 2 and 3. However, the resultant deeper collapse panel occupies a larger internal volume of the container, and the overflow capacity of the container is significantly decreased. In order to compensate for this decrease in overflow capacity, the container diameter must be increased. Any increase in container diameter, however, decreases container rigidity. Thus, any container rigidity gained by increase in the size of the collapse panel is offset by the need to regain the lost overflow capacity. The present invention eliminates the aforementioned disadvantages.